Beam scanning dissector



Patented-July 19, 1938 UNITED STATE BEAM SCANNING DISSECTOR Philo T. Farnsworth, San Francisco, Calif.) assignor to Farnsworth Television Incorporated, San

Francisco, Calif., a corporation of California Application November 2, 1936, Serial No. 108,724

Claims.

of the electron storage systems of scanning the object to be pictured, but without the disadvantages of those types of scanning device which utilize photoelectric surfaces formed from isolated droplets or mosaics of emissive material.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

According to my invention, 1 form a current image of the picture to be transmitted and direct this against a suitable target, to form thereon a charge pattern which is then scanned by a cathode ray beam to release the charges on elementary areas of the target and so produce a television signal.

The invention also consists in apparatus for carrying out the above method and details and arrangements in connection therewith, as will be apparent from the description given hereafter.

The drawing is an axial sectional view of a tube embodying a preferred form of the apparatus of my invention, the auxiliary circuits, connections and apparatus therefor being shown schematically.

' Carrying my invention into eflectjn one convenient manner, I provide an evacuated envelope I, usually of glass, one end of which is provided with a window 2, through which an: image of a photoelectric apertured screen 4. This apertured screen'may well be made of silver wire which is, during the course of processl 8.. ox- 55 idizcd and sensitized with'various photoelectric the object to beranalyzed is projected by means of lens system 3, and this image is focussed on materials, preferably caesium. Between the photoelectric screen 4 and the window 2, I prefer to locate two additional and parallel electrodes, a collecting screen 5, ands charge storage electrode 6. The collecting screen may be of relatively large mesh as compared to the mesh of the photoelectric screen 4. The charge storage electrode 6 is preferably formed of a thin sheet of mica or other thin dielectric, such as glass or quartz. This electrode is backed on the side facing the window 2 with a transparent or semi-transparent coating I of-a conductor, and contact is made thereto so that an exterior lead 8 may be used to place a charge thereon. This coating may be a thin, sputtered coating of silver platinum, nickel, etc., as is well known in the art. The three electrodes, photoelectric screen 4, collecting screen 5, and charge storage electrode 6, are placed relatively rounded on the outside of the envelope I by a focusing coil 9, energized by a focusing source I0 under the control of the resistor I i. opposite end of the tube I prefer to mount an electrode gun on a re-entrant stein II, this gun comprising an electron emitting cathode i4 held at a negative potential to ground by source it,

close together, and are sur- At the and an apertured anode IS, the two cooperating to produce, when energized, an electron beam of small cross-section. I also prefer to provide a metalized coating IS on the wall of the tube between the gun anode and a point adjacent the photoelectric screen 4, this coating being connected to the anode I5 through a link ll.

Exteriorly of the tube a cathode source It! is utilized to heat cathode l4,. and a gunanode source it is connected to the anode i5 and film l8. Scanning .oscillators. 20 and II, preferably giving a saw tooth wave form, energize magnetic scanning coils 22 and 23, positioned so that their fields will cause the beam emitted from gun anode l5 to scan successive elementary areasof a picture area on charge storage electrode 6, passing through the intermediate screens. The transparent coating 1 on charge storage electrode 8 is connected through an output resistor 24 to ground through an output battery 25, the negative end of which isconnected to the resistor. Collecting electrode 5 is grounded, whereas the photoelectric cathode 4 is connected to ground through a photo-source 28, the negative end of which is connected to the photoelectric screen.

It will be obvious that other connections will serve the purpose, the main object being to maintain the collecting screen 5 more positive than and through-collecting screen trons emitted are collected either of the adjacent electrodes whereby it may act as an accelerating electrode, and to have the output resistor 2 in a position where it can register differences of potential between the translucent film l and accelerating electrode collecting screen 5.

In operation, the image of the object to be analyzed is focussed on photoelectric screen d and low velocity electrons are emitted therefrom. These electrons constitute an electron image which is accelerated toward charge storage electrode 6 by the action of the accelerating electrode 5, and as a result of the low velocity electron image striking the insulating surface of the charge storage electrode, there is deposited thereon a corresponding charge pattern; and the displacement current through the translucent film on this electrode, due to the accumulation of this charge, causes a constant current component to flow through the'resistor 2d, from which an output lead 28 may be connected to an amplifier in accordance with the usual practice. Simultaneously with the foregoing action, the electron gun, energized, for example, with a 4000 volt anode potential, directs a beam of high velocity electrons through photoelectric screen it 5, to finally land against the surface of the charge storage electrode 8, on which is already deposited the charge image. The point of impact of this beam emits secondary electrons and thus swings positive to a definiteequilibrium value.- The secondary elecon the collecting screen 5, which is more positive, as stated above, than the other two associated electrodes.

As the beam from the gun is scanned over the charge image, each successive area of the charge image is thus brought to its positive equilibrium value of potential. This destroys the image character of the aggregate charges on the surface -of the charge storage electrode, successively releasing the positive charges on the translucent film bound by sulating suri'ace,-and causing a pulsating current through the resistor in the opposite direction to the constant component due to the accumulation of these charges. The charge on each elementary area of the charge storage electrode at the instant before it is scanned is proportional to the product of intensity of the light on the corresponding element of the cathode and the time which has elapsed since it was last scanned, and, since the intervals between scansion are uniform, it follows that the 'charge released from each element of the charge storage electrode when scanned is directly proportional to the illumination of an elementary area of the picture to be transmitted, and hence the voltages appearing across output resistor 26 will constitute an analysis or, if desired, a television signal which may be amplified and transmitted in the usual manner.

I claim:

1. A beam scanning envelope containing a dissector comprising an planar apertured photoelectric member, a thin transparent screen of.

insulating material positioned parallel to said member on one side thereof, a thin translucent and conducting coating on the face of said screen away from said member, means for projecting an optical image through said coating and screen onto said member to cause electronic emission therefrom, means for directing said electrons against the uncoated face of said member to form a-charge image thereon, a cathode and the negative charges on the in- I anode cooperating to project an electron beam of elementary cross-section through said member to impact the uncoated face of said screen carrying said charge image, said beam containing electrons of sufficient velocity to create secondary electrons upon impact with said screen whereby said charge image is neutralized, and means connected to said coating for utilizing changes in the charge on said uncoated face.

2. A beam scanning dissector comprising an envelope containing a planar apertured photoelectric member, a thin transparent screen of insulating material positioned parallel to said member on one side thereof, a thin translucent and conducting coating-on the face of said screen away from said member, means for projecting an optical image through said coating and screen onto said member to cause electronic emission therefrom, means for directing said electrons against the uncoated face of said member to form a charge image thereon, a cathode and anode cooperating to project an' electron beam of elementary cross-section through said member to impact the uncoated face of said screen carrying said charge image, said beam containing electrons of suflicient velocity to create secondary electrons upon impact with said screen, an open mesh electrode between said screen and said member, and an output circuit connected between said coating and said electrode.

3,-A beam scanning dissector comprising an envelope containing a planar apertured photoelectric member, a thin transparent screen of insulating material positioned parallel to said member on one side thereof, a thin translucent and conducting coating on the face of said screen away from said member, means for projecting an optical image through said coating and screen onto said member to cause electronic emission therefrom, means for directing said electrons against the uncoated face of said member to form a charge image thereon, a cathode and anode cooperating to project an electron beam of elementary cross-section through said member to impact the uncoated face of said screen carrying saidcharge image, said beam containing electrons of sufiicient velocity to create secondary electrons upon impact with said screen whereby said charge image is. neutralized, means connected to said coating for utilizing changes in the charge on said uncoated face, and means for collecting emitted secondaries.

4. A beam scanning dissector comprising an envelope containing a planar apertured photoelectric member, a thin transparent screen of insuimpact the uncoated face of said screen carryingsaid charge image, said beam containing electrons of sumcient velocity to create secondary electrons upon impact. with said screen whereby said charge image is neutralized, means connected to said coating for utilizing changes in the charge on said uncoated face, and means for directing said beam over successive elemental areas of thecharge image on said screen.

5. A beamscanningdissector comprising an envelope having therein a planar apertured photoelectric member, a thin transparent screen of insulating material positioned parallel to said member on one side thereof, a thin translucent conductive coating on the face of said screen away from said member, means for projecting an optical image through said coating and screen onto said member to cause electronic emission l therefrom, means for directing said electrons against the uncoated face of, said member to form a charge image thereon, an electron gun arranged when energized to project a cathode ray beam of elementary cross-section through said memberto impact the uncoated face of said screen carrying said charge image, said beam- -having suilicient velocity to create secondary electrons upon impact with said screen in number sumcient to neutralize said charge image, means connected to saidv coating for utilizing changes in the charge on said uncoated face, and

means for directing said beam over successive elemental areas of thecharge image on said 10 screen.

PHILO '1. FARNSWORTH. 

